Wire-guided anti-tank missiles are known in which an operator observes a target tank using an imaging system, typically a thermal imaging system, and aligns a graticule with the target. The missile is then fired and guided to the target, either by the operator or by an automatic guidance system, by means of guidance signals fed to the missile along a wire which the missile unwinds behind itself and leaves stationary on the ground as it moves toward the target. Such a missile guidance system is described in British Patent Specification No. 2,087,185A.
It has been proposed to replace the wire link by a radiation link comprising a source of radiation at the missile launch point which can be modulated with the guidance information. The radiation is formed into a beam of angular width covering the target and the expected range of lateral movement of the missile on its way to the target. The missile comprises a rearwards facing radiation collection and detection system which receives the modulated radiation from the source and converts it into appropriate guidance signals for the missile flight path control system.
Desirably, the radiation source should produce non-visible radiation for obvious security reasons and should also be infra-red radiation of a sufficiently long wavelength that the radiation can penetrate mist, fog or smoke as much as possible. A typically convenient source of radiation is the carbon dioxide laser which operates at a wavelength of 10.6 .mu.m. Cooled infra-red detectors are then generally desirable to detect the radiation with adequate sensitivity. Stored electrical energy or compressed gas is then required within the missile to provide detector cooling in a form to be instantly available on firing the missile but to have a long storage life. Unavoidably the stored energy is limited and therefore detector cooling cannot start until the missile is fired and there will be a delay before the detector responds to radiation and while the detector temperature falls to its operating value.
From the instant of firing the distance between the missile and the source increases, growing more rapidly as the missile accelerates under booster rocket power to its cruise velocity. Since the received radiation energy at the missile will fall inversely as the square of missile distance, the received energy will fall rapidly, exacerbating the effect of the delay in detector sensitivity rise.